Current practice in mobile radio transceivers is to derive the operating frequencies with phase locked loop synthesizers. In such a synthesizer, a phase locked loop circuit controls a variable frequency oscillator, i.e. a voltage controlled oscillator (VCO), to lock in at the frequency and phase angle of a standard or reference frequency. In this fashion, the VCO will have the same accuracy as the standard. The phase locked loop operates to track the operation of the reference oscillator.
In general, a phase locked loop includes a VCO, a reference oscillator, a phase detector and a loop filter. The phase detector compares the phase of the VCO output signal with the phase of the signal from the reference oscillator. The phase detector generates an output signal related to the difference between the two compared signals. This output signal is processed by a low pass filter and is then applied as a control signal to an input terminal of the VCO to control the frequency of the oscillator output signal.
Modulation of the VCO output signal has heretofore typically been accomplished by applying a modulation signal to either the reference oscillator, the VCO or to both the reference oscillator and the VCO. If the reference oscillator is modulated, the modulation response of the phase locked loop has a transfer function which is relatively flat for low frequencies up to approximately the natural resonant frequency of the phase locked loop. The transfer function, however, decreases rapidly at approximately the natural frequency of the phase locked loop. It is therefore generally impractical to use this method of modulation for frequencies greater than the natural frequency of the loop because of the rapidly decreasing loop response at such frequencies.
Such a method is not a satisfactory approach for frequency modulating a VCO in typical FM transmitter applications due to the lack of a high frequency response. The phase locked loop behaves as a low pass filter with respect to modulation applied to the reference oscillator. The ability of the VCO output to track the reference source frequency is limited by the effective loop gain of the feedback control loop.
Alternatively, if the VCO is modulated directly, then the loop behaves as a high pass filter with respect to applied modulation. The modulation response while relatively flat for higher frequencies decreases rapidly for low frequencies, thereby exhibiting a less than satisfactory modulation response characteristic.
In order to achieve wideband modulation, the frequency spectrum of the modulation input must substantially exceed the loop bandwidth of the frequency tracking phase locked loop. In, for example, mobile radio applications, the loop bandwidth is usually less than 100 Hz, while the modulation input may extend from a few Hz to 5 KHz.
One approach that has been employed to achieve wideband modulation is to modulate both the VCO and the reference oscillator in a complementary fashion. In this regard, by modulating both the reference source and the VCO in the proper ratio, a flat modulation response, i.e., a constant modulation level over a wide frequency band, can be obtained.
The viability of a particular method of modulating a phase locked loop is highly application dependent. In this regard, if the phase locked loop bandwidth is large, when compared with the modulation input, when the reference oscillator should be modulated. If it is desirable to handle the audio spectrum which ranges from 200 Hz, to 4 KHz, and if the phase locked loop is able to track to 1 KHz, then the loop will track up to the 1 KHz level and then be unable to satisfactorily track frequencies above 1 KHz. Under such circumstances, it may be possible to modulate the reference oscillator to achieve satisfactory operation.
On the other hand, if the phase locked loop only has a tracking bandwidht of 100 Hz, then the reference oscillator can not be modulated to achieve satisfactory system operation. Thus, based on the limitations of a particular system in a given application context, modulation of the reference oscillator, the VCO or both may be in order.
In many applications, it is undesirable or impossible to modulate the reference source. In this regard, it is often desirable to make the reference oscillator more rock solid stable with respect to its major intended purpose--namely, accurately defining a reference frequency onto which the phase locked loop can lock. This increased stability requirement is inherently inconsistent with an ability to readily modulate or change the frequency of the reference oscillator.
In other applications, the reference frequency source may be a frequency standard which is not accessible. Thus, in such a system which does not include a reference oscillator, it is impossible to modulate the reference source.
In a related patent application, Ser. No. 244,498, Johannes Vandegraaf entitled "PHASE LOCKED FREQUENCY SYNTHESIZER WITH SINGLE INPUT WIDEBAND MODULATION SYSTEM", and filed concurrently herewith, a flat, i.e., frequency independent, modulation of the VCO via a modulation input port of the VCO, is achieved using a dual integration, pre-equalization method for VCO modulation which is described in detail therein. In this copending application, an inverse analogue of a predetermined control loop transfer function is interposed between a modulation source and the VCO in order to achieve wideband modulation.
While this approach to providing wideband modulation has various advantages, it requires a large dynamic range that may be excessive for some applications. For example, in a hand held radio, supply voltages are limited to, for example, 4 volts. If, in order to provide the necessary low frequency boost, a dynamic range is required of, for example, 100 to 1, then the input signal range must be capable of varying from 0.04 volts to 4 volts. Because of problems due to noise, it may be difficult for such a radio to handle such low signal levels. Thus, in applications where supply voltages are limited, the relatively large dynamic range required in the above related application may lead to some difficulties.
The present invention relates to a method and apparatus that achieves wideband modulation that requires less low frequency boost and a smaller dynamic range than the solution in the applicant's copending application. As in the copending application, a modulation equalization network is associated with the phase locked loop to thereby result in frequency independent modulation of the VCO. However, in the present invention, rather than interposing the equalization network between the modulation source and the VCO modulation input, the equalization network is disposed to introduce the modulation signal into the control loop ahead of the loop filter but after the phase detector output.
The interposed equalization network, in contrast to many prior art systems, requires only a single input port to achieve a flat modulation response thereby lending itself to a modular implentation. Additionally, in the present invention, the modulation processing is performed totally outside the phase locked loop. The network serves to provide a boost to low frequency signals from the modulation source using an integrator network while providing a high frequency response that is essentially the inverse of the phase locked loop filter response.
The present modulation system is a single input system which creates a flat frequency response down to very low frequencies and permits wideband modulation of the VCO. Thus, in contrast to many prior art attempts to provide wideband modulation of the phase locked loop, the present invention does not require separate modulation inputs at different points in the control loop.